[stm32l] linker script was duplicating sections

Bad includes are bad.
[stm32-ocm3] Improve documentation and defaults
[stm32-ocm3] remove redundant linker scripts

Don't need these, we can refer directly to linker scripts in the OCM3
directory.
[stm32-ocm3] reduce warnings in valid cases

You should normally provide a LINKERFILE, or LINKERSCRIPT, but if you
don't provide one, and do provde SUBTARGET, we'll do our best to find
the right one.
[stm32-ocm3] Expand clock module implementation

In certain cases, delaying may be off by up to an entire uSec, when
the systick counter is about to wrap.  The implementation here is not the
most precise, with perfectly counted assembly loops, but it's clear and
functional.  You shouldn't be using these low level routines for anything
critical anyway.